refactor: remove anyhow from channels, group agent fields, gate qdrant

[bug-ops]

Summary

• Remove anyhow dependency from zeph-channels, replace with typed ChannelError mapping
• Group Agent struct 30+ flat fields into 5 inner structs: MemoryState, SkillState, ContextState, McpState, IndexState
• Add workspace-level qdrant feature flag to allow opting out of qdrant-client compilation

Closes #288, closes #289, closes #290
Part of #282

Test plan

• cargo build
• cargo clippy --workspace -- -D warnings
• cargo nextest run --workspace --lib --bins (1351 tests pass)
• Performance review: no regression
• Security audit: no issues, cargo-deny clean

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.22222% with 156 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/zeph-core/src/agent/learning.rs	6.52%	43 Missing ⚠️
crates/zeph-core/src/agent/mod.rs	61.00%	39 Missing ⚠️
crates/zeph-core/src/agent/context.rs	53.62%	32 Missing ⚠️
crates/zeph-core/src/agent/mcp.rs	17.24%	24 Missing ⚠️
crates/zeph-channels/src/telegram.rs	10.00%	9 Missing ⚠️
crates/zeph-core/src/agent/persistence.rs	11.11%	8 Missing ⚠️
crates/zeph-core/src/agent/index.rs	83.33%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (42.22%) is below the target coverage (60.00%). You can increase the patch coverage or adjust the target coverage.

@@            Coverage Diff             @@
##             main     #294      +/-   ##
==========================================
- Coverage   80.20%   80.03%   -0.18%     
==========================================
  Files          98       98              
  Lines       23855    23926      +71     
==========================================
+ Hits        19134    19149      +15     
- Misses       4721     4777      +56     

Files with missing lines	Coverage Δ
crates/zeph-core/src/agent/streaming.rs	83.19% <100.00%> (ø)
src/main.rs	64.58% <ø> (ø)
crates/zeph-core/src/agent/index.rs	48.64% <83.33%> (ø)
crates/zeph-core/src/agent/persistence.rs	4.65% <11.11%> (-0.17%)	⬇️
crates/zeph-channels/src/telegram.rs	34.26% <10.00%> (+0.27%)	⬆️
crates/zeph-core/src/agent/mcp.rs	11.25% <17.24%> (+0.24%)	⬆️
crates/zeph-core/src/agent/context.rs	85.79% <53.62%> (-1.31%)	⬇️
crates/zeph-core/src/agent/mod.rs	82.28% <61.00%> (-0.59%)	⬇️
crates/zeph-core/src/agent/learning.rs	22.13% <6.52%> (-0.98%)	⬇️

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